First, let's try to define the next definition of "micro hole drilling." However, like many metal cutting terms, it is often difficult to give a strict and precise definition. The metal cutting industry is full of relativity. In fact, almost all discussions aimed at determining the value of metal cutting parameters will be "open or back depending on the specific processing conditions."

 

However, there is an industry insider – vice president of sales and engineering at Big Kaiser, and Jack Burley, distributor of Sphinx’s product line for Swiss micro drill bit makers – willing to use a figure to define micro hole drilling. He explained that Sphinx defines the hole drilling in the range of 0.05-2.5mm as micro hole drilling. For ease of discussion, we agree with this definition for the time being. Burley also added that this standard is not static, and with the continuous improvement of processing technology and materials used to make micro-drills, the minimum hole-making lower limit of micro-hole drilling will continue to shrink.

 

What is microfabrication?

 

Microfabrication is not necessarily the same as machining micro-parts. Although in the medical and electronic device manufacturing industry, there is indeed a trend of increasingly miniaturized parts, on many large-sized workpieces, it is often necessary to process some very small features. Such processing can only be accomplished by microfabrication techniques.

 

Burley pointed out, "I have seen some workshops that routinely use drills with a diameter of 6.35mm. I just changed the tool to a drill with a diameter of 1.52mm and tried to make microholes. When the machining could not be completed, they did not know the problem. The problem is that when you break through the 2.5mm aperture limit, the accuracy requirements will increase dramatically.Accordingly, the machining process must be fully optimized.This is not a problem that can be solved by changing tools alone, only trained. The shop can do this conversion."

 

Machine tool equipment can achieve micro-drilling?

 

Micromachining requires the machine's feed axis to have a sufficiently high sensitivity and a sufficiently small resolution. The machine must also have a highly accurate spindle that can be rotated at high speeds with very little dynamic path. Dynamic jump error is an important measure of high-speed spindles that can be measured using laser or capacitive impedance sensing technology.

 

Burley explained, “The machine tool builder has developed a special machine tool for micromachining. For those workshops that plan to use existing equipment for microfabrication, I recommend asking experts to evaluate the machine tool to ensure that it can achieve sufficiently fine feeds. And the dynamic radial error (TIR) ??is less than 0.0025mm."

 

How to hold the micro drill?

 

There are two basic types of micro-drills: (1) Shank bit: its shank diameter is the same as the diameter of the drilling part (Figure 1 left); 2 standard shank bit: regardless of the diameter of the drilling part, the handle diameter is the standard Size φ3mm (in Figure 1, right). The silk shank bit has a low manufacturing cost and therefore has a price advantage, but in order to hold different diameter drill bits, chucks of different sizes are required. In addition, due to the smaller outside diameter of the shank bit, the operation is more difficult.

 

    Regardless of the type of drill used, toolholding technology is very important for microhole drilling. The micro drill has strict requirements for radial jump accuracy. Any radial jump error in the machine tool spindle, tool chuck, and chuck can have a large effect on the micro drill bit. For micro drills less than 2 mm in diameter, the maximum run-out error (TIR) ??measured at 4 times the hole depth should be less than 0.0025 mm.

    

     Burley explained that "the tool chuck and the machine spindle constitute a support system to ensure that the drill tip can drill into the workpiece as accurately as possible. Our tests show that high precision chucks that meet the AA class standards can provide stable jump accuracy. This makes the tool change easier. For ultra-small diameter drilling on a machining center, the chuck is usually manually clamped in the spindle to eliminate the positioning error caused by the automatic tool changer."

 

    What is the workpiece material?

 

In order to reduce the force required for chip removal and to prevent cold welding between the chip and the chip flutes, it is required that all cutting edges of the drill be worn uniformly, which is essential for micro hole drilling. Therefore, the surface finish of the micro drill must be very good.

 

Micro hole drilling tools must match with the material being processed, which is no different from conventional drilling. The difference is that the cutting edges of micro drills must be very sharp and have a good centering, because these micro tools lack sufficient structural integrity to rely on thrust to drill the workpiece.

 

Burley explained that “microtool manufacturers can optimize the micro drill bit's machining performance by using different geometrical edge shapes and tool materials. By changing the chisel edge thickness, we can enhance or weaken the drill bit strength according to the workpiece material. Although high-speed steel is the most commonly used drill bit material, nano-crystalline carbide drills have the longest tool life.I recommend that the machining shop work closely with the tool supplier to create cutting parameters and tool specifications based on the specific machining, because each The tools made by each tool supplier are slightly different."

 

What is the tolerance requirement?

 

For many micro drills, there is no lack of ability to drill such small holes. However, with the wide application of micro-hole drilling, people are increasingly demanding that micro-hole drilling can achieve similar dimensional accuracy and surface finish as conventional drilling.

 

Burley pointed out that "in micro hole drilling, the accuracy of the drill bit is critical. Since micro hole drilling is basically a 'one-stroke deal', it is best to reach the size requirements in one feed. To cover the required size range, We provide micro drills, regular drills and center drills in increments of 0.01mm."

 

Burley went on to say, “More stringent dimensional tolerances have driven the development of precision hole machining tools. Micro files and reamers are used to further improve the hole wall surface finish (usually 20 rms) produced by micro drills, as well as the dimensional accuracy of the holes. The straightness is increased to a level that cannot be achieved by drilling alone.As long as the micro hole drilling process elements (feed control, spindle and tool chuck diameter jump error, tool material and geometry, cutting parameters, etc.) To meet the requirements, tolerances of 0.0025mm can be achieved through microhole boring and reaming."

 

      What is the depth of the hole?

 

With the continuous advancement of cutting tools and processing technology, the level of machining of deep holes with micro drills is also increasing. A centering hole can be pre-drilled for deep hole drilling using a center drill with a reverse diameter tolerance (ie +0.004/-0 mm) instead of a standard tolerance in the positive (negative) direction. This procedure enables the micro drill bit with a slender, brittle, and easily broken structure to drill into a hole with a slightly larger diameter to prevent the drill bit from being constrained and broken.

 

Burley said, "We have successfully drilled holes with a length-to-diameter ratio of 10 to 15. In addition to using a centering hole to guide the drill, we also recommend a 'slam' machining program. For holes with a depth of 2 times the diameter, I recommend using the 'slamming' procedure with 3 infeeds and 1 outfeed. For every 2 in. increase in hole length, an additional feed is added and the entire tool is removed from the hole. To complete this process, Machine tools with precise servo control are very important."

 

He added, “Because the chips formed by the micro drill bit are small, the cutting heat input to the chip is not much. Therefore, it is necessary to take measures to clear the chip and cool the drill bit. We recommend using oil mist/compressed air to assist chip removal and cooling the drill bit. ”

 

How to drill into the workpiece?

 

Ideally, the micro drill should be able to drill vertically into the workpiece. However, not every hole can satisfy this condition - the surface of the orifice of some workpieces is inclined at a certain angle.

 

Burley pointed out that if the angle at which the drill bit is drilled is less than 10°, careful feeding can prevent the bit from slipping on the surface of the workpiece and successfully complete machining. If the inclination is greater than 10°, some measures need to be taken to solve the problem of bit slippage. For example, a small notch can be milled on the slope of the drilling point, creating a vertically drillable platform for the center drill, and then drilling through the centering hole with a micro drill bit.

 

Another solution for bevel drilling of micro drills is to use a CNC machine with a B-axis. First, the spindle drives the drill bit feed (the drill point is perpendicular to the drill point of the workpiece). Once the drill bit contacts the workpiece, the B axis rotates at a given tilt angle and the drill bit can be drilled vertically. With this method, there is no need for pre-drilling centering holes.

 

How many holes need to be machined?

 

The next question that Burley proposed to the machining shop that plans to work on micro-hole drilling is: How many holes need to be machined on the workpiece? Micro-hole drilling can be very effective for high-volume machining (either drilling on many parts or drilling many holes on each part).

 

Burley explained that "the processing volume of the hole is different, the micro hole drilling process specification may also be changed. If the processing volume is large, the process optimization should focus on improving processing efficiency and extending tool life. For example, if possible, Pre-drilling and centering holes should be eliminated in order to speed up production cycle, and coating bits that can reduce tool wear can be used. Generally, due to the small size, most of the miniature tools do not use coatings. Thickness may affect the nominal size of the micro-tools.For micro-tools used for high-volume machining, any coating used must be very thin (less than 0.001mm thick) and very smooth.”

 

What is the skill level of the workshop staff?

 

Perhaps as important as all the above issues are: Does the workshop staff adapt to the transition to microfabrication? Not only do employees need to master new processing skills, they also need to know that these miniature tools, which are thinner and fragile than hair, must be handled with care and care.

 

Burley said, "Processing skills can be learned. I have seen some operators who work on micro-fabrication, they know when the tool will fail, or when it will become blunt. They may not be able to tell the truth, depending on the rich This is a valuable skill, and I think that in addition to investing in the hardware needed for processing, the workshop manager must also train the operator to master the specific skills required for micromachining.Use miniature tools with a diameter of less than 2.5mm. Cutting is a different experience."

 

    The circular part shown in Figure 5 is a filter used in the medical industry with 400 tiny holes that are barely visible to the naked eye. The processing conditions of these micropores are: Workpiece material: 6Al4V titanium alloy; micropore size: aperture 0.15mm, hole depth 0.8mm; number of holes: 400; processing machine: Kern precision microhole machining machine; spindle speed: 6,300 r /min; feed rate: 0.0025mm/r. Due to the large number of holes, in order to shorten the processing cycle, the pre-drilling centering hole process was eliminated, and the “slamming” drilling method was not used.